The quartz crystal has been used for frequency control and precision time-keeping for decades. From the beginning of the crystal industry, engineers have been trying to develop manufacturing techniques which would relative the labor-intensive manufacturing process of cutting, lapping, polishing, plating, and tuning.
A second driving force in the crystal industry has been the need for miniaturization. With the continued efforts of the semiconductor industry to reduce device geometry and increase density, there is an increasing demand to reduce the size of the clocks and ultimately the quartz resonator itself. This has resulted in many new miniature crystal designs over the past twenty years.
In most of these earlier applications, the crystal was mounted in its own container. Fabrication techniques were developed in the seventies which allowed the placement of the crystal resonator and the active circuitry components in the same container. These devices were typically for clock oscillator applications and the circuitry was fabricated utilizing thick film processing techniques. Hybridization offered reduced production costs, greater circuit density and reduced the profile of the package, and at the same time allowing all environmentally sensitive components to be hermetically sealed in a common package.
These processing techniques also presented some problems which had to be overcome. Some of the encapsulates used to protect the bonds on the die caused problems with the crystal aging characteristics due to the long term outgassing of the materials and subsequent contamination of the crystal surfaces. A second difficulty was encountered in mounting the crystal to the alumina substrate. Lead ribbon wire was formed on one side to be reflowed to the alumina substrate and make electrical connection and support the crystal at the other end. The crystal was then typically cemented to the support using conductive epoxy. This often developed strains in the crystal blank and caused problems with frequency stability over the operating temperature range.
A major step forward was the development of the tuning fork crystal. Such forks were produced using photolithographic processing and wet chemical etching. Metallic coatings consisting of a thin layer of chromium and a thick layer of gold were placed on the surface of the quartz by vacuum deposition techniques. The surface wa then coated with photoresist and patterned to the desired geometry. This photoresist mask was then used to define the pattern in the metallization. The quartz, with the areas to be etched defined in the metal masks, was then placed in a heated etchant primarily consisting of hydrofluoric acid to chemically mill out the resonators. These processing techniques allowed batch processing of many resonators at a time, and substantially reduced production costs and resonator size. However, while plural resonators all part of a single oscillator may have been fabricated in a single quartz substrate, the resonators could not be completely isolated from one another both mechanically and electrically. Thus, plural oscillators were not formed in a the same quartz substrate since they could not operate independently of one another and their frequencies could not be accurately controlled.
There is a need, then, for a technique of fabricating plural crystal oscillators on a single quartz substrate, to promote miniaturization and facilitate manufacturing.
A further disadvantage in the prior art is that crystal oscillators formed on different substrates may have slight differences in operating characteristics despite identical fabrication techniques. This is undesirable where plural oscillators are to cooperate with one another in a circuit application where their characteristics should be matched. By way of illustration, it has been known for years that bridge or balanced type instrument configurations have an inherent stability that other types of circuits do not have. Consider, for example, the differential amplifier at the front end of an operational amplifier. Both the left side and right side transistors of the differential amplifier are manufactured in similar environments with similar doping profiles. Consequently, they are as close to being identical as possible. The input signal is applied between the two bases, and the output signal is taken between the collectors of the transistor pair. This balanced arrangement of two identical transistors results in an amplifier whose characteristics are immune to common-mode interfering signals, temperature effects, supply voltage changes, external noise, etc. For operational amplifiers, the stability of the balance is expressed in terms of the common-mode rejection ratio.
An AT-cut quartz resonator has a temperature coefficient that is a function of the angle of cut. The curves of FIG. 1 indicate the dependence of this coefficient on the angle, and show clearly that if two discrete resonators are to track over a range of temperatures, their angles of cut must be nearly identical. Thus, it has been difficult to obtain the full benefit of the balanced bridge design in circuits involving several quartz resonators, because such great attention must be paid to control both the angle of cut and the operating temperature.